The first thing to know about Combat, is that without a weapon equipped, you won't last long!

Now, there are two types of weapons:

Melee Weapons (ie. Swords, Axes, Staves, Clubs, etc...)
Ranged Weapons (ie. Bows, Crossbows, Slings, etc...)
You are allowed to equip both a Melee and a Ranged weapon at the same time, however you can only wield one at a time. To switch rapidly between an equipped Melee and an equipped Ranged weapon, use the Combat Toggle button:

Combat Toggle - Pressing this button will cause the character to toggle between equipped melee and range weapons. Of course both a ranged and a melee weapon must be equipped for this to work.
When equipping melee and ranged weapons, keep in mind the 'hands' of the weapons must match. The following combinations work: